Light metal-sulfur organic electrolyte cell

ABSTRACT

A LIGHT METAL ANODE-SULFUR CATHODE CELL EMPLOYING AN ORGANIC ELECTROLYTE IS PROVIDED WITH IMPROVED CELL SEPARATOR MEANS. ION EXCHANGE MEMBRANES EXHIBITING COMPATIBILITY WITH ORGANIC SOLVENT, AND HAVING HIGH EXCHANGE CAPACITY AND HIGH ELECTRICAL CONDUCTIVITY HAVE BEEN DETERMINED TO BE SUITABLE FOR USE AS CELL SEPARATORS AND TO HAVE SUCH PHYSICAL AND ELECTROSTATIC CHARACTERISTICS AS TO PROVIDE IMPROVED CELL UTILIZATION EFFICIENCY AND EXTENDED SHELF LIFE.

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ATTOR EY United States Patent O 3,806,369 LIGHT METAL-SULFUR ORGANICELECTROLYTE CELL Arabinda N. Dey, Needham, and Per Bro, Andover,

Mass., assignors to P. R. Mallory & Co., Inc., Indianapolis, Ind.

Continuation of abandoned application Ser. No. 775,444, Nov. 13, 1968.This application Feb. 28, 1972, Ser. No.

Int. Cl. II0lm 35/00 U.S. Cl. 136-6 LN 10 Claims ABSTRACT OF THEDISCLOSURE A light metal anode-sulfur cathode cell employing an organicelectrolyte is provided with improved cell separator means. Ion exchangemembranes exhibiting compatibility with organic solvents, and havinghigh exchange capacity and high electrical conductivity have beendetermined to be suitable for use as cell separators and to have suchphysical and electrostatic characteristics as to provide improved cellutilization eiciency and extended shelf life.

BACKGROUND OF THE INVENTION Field of the invention This inventionpertains to high energy density cells employing light metal anodes andsulfur cathode members in organic electrolytes, and more particularly tocells of this type incorporating ion selective membranes as cellseparators.

Description of the prior art This application is a continuation of S.N.775,444, filed Nov. 13, 1968, which is now abandoned.

In S.N. 536,814 now matured into U.S. Pat. No. 3.413,- 154, assigned tocommon assignee, there is disclosed a high energy density cell comprisedof a light metal anode, a sulfur cathode, an organic electrolyte and athin barrier of microporous inert material The organic electrolyte ofthe cell is composed of an organic solvent in which there is dissolved alight metal or ammonium salt containing a tetrafluoroborate,tetrachloroaluminate, perchlorate or chloride anion.

While the sulfur cathode of such cells is not itself soluble in theorganic electrolyte, certain discharge products of the cathode aresoluble. Thus, it has been observed that during shelf life and operationof this type of cell there are formed, for example, soluble polysuldes,8x2, x=29.

Diffusion of these polysuldes in the cell electrolyte is notparticularly detrimental to cell shelf life r utilization, if migrationthereof from cathode to anode is prevented.

Thus, if soluble discharge products are confined and remain in proximityto the cathode, the collective mass comprised of the cathode and itssoluble discharge products will permit cell utilization of a high order.

In the light metal sulfur-organic electrolyte cells of theabove-mentioned application, it has been observed that some migration ofthe soluble polysuldes away from the cathode occurs. The barrier of thecells is comprised of a microporous member adapted only to reduce freeelectrolyte flow to a negligible rate. While effective to provide asubstantial physical impediment to migration of all ionic species in thecell and a resultant confinement of polysulides, the barrier is notselectively effective to retard deleterious migration such that maximumcell utilization may be attained.

3,806,369 Patented Apr. 23, 1974 ICC SUMMARY OF THE INVENTION Thepresent invention provides improved shelf life and utilization eiciencyin cells comprising a light metal anode, a cathode, the active materialof which is sulfur, and an electrolyte consisting of an organic solventcontaining a salt by introducing therein a cell barrier member orseparator capable of selectively impeding the migration of certain ionicspecies in the cell. In particular, barrier means providing bothphysical and selective electrostatic retardation of migration isincorporated in the cell in the form of an ion exchange membraneseparator.

In the present invention it has been discovered that certain membranes,designed for commercial use in aqueous mediums for such purposes aswater demineralization, dealkalization and softening, and comprised ofsynthetic organic resins comprising cross-linked polyelectrolytes havinglarge numbers of ion active groups attached thereto, are effective toprovide the requisite physical impediment to migration and further areoperative to substantially reduce if not totally inhibit the migrationof soluble cathodic discharge products in light metal-sulfur organicelectrolyte cells.

In the present invention there has been determined further thefeasibility of the use of membranes of cationic nature to retard themigration of soluble polysulde discharge products.

It is a primary object of the present invention to provide a lightmetal-sulfur organic electrolyte cell having improved shelf life andutilization eiciency.

It is a further object of this invention to provide a light metal-sulfurorganic electrolyte cell incorporating means for physically andelectrostatically rearding the migration of soluble ionic activematerial.

It is an additional object of this invention to provide a high energydensity cell comprising a light metal anode, a cathode the activematerial of which is sulfur, an organic electrolyte and an ion exchangemembrane cell separator.

The foregoing and other objects and features of the invention will beevident from the following detailed description of the invention and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of acell arranged in accordance with the invention illustrating the cellbarrier structure and ionic species present in the cell.

FIG. 2 is a graphic display of the operating characteristics of cellsconstructed in accordance with the invention in comparison with those ofa like cell employing a glass filter paper separator as the cell barriermember.

DESCRIPTION OF A PREFERRED EMBODIMENT In the schematic drawing of FIG.1, there is set forth a cell 10 comprised of a lithium anode 12 and asulfur cathode 14 immersed in an organic electrolyte 16. Also disposedin contact with the electrolyte is a barrier element 18 so arranged inthe cell as to separate the anode and cathode members. In thisconstruction of a practical cell, barrier 18 is placed upon cathode 14and there is interposed between anode 12 and barrier 18 an absorbentelement which is saturated With the electrolyte. The absorbent is placedin direct contact with its interfaced cell components such that theelectrolyte is in electrical contact with the anode and the barrier.Electrolyte contact with the cathode is restricted by the microporosityof the barrier to an extent that free ow of electrolyte therethrough isreduced to a negligible rate. In operation of the practical cell,however, both electrodes of the cell .vents for use in the cell comprisepropylene carbonate,

gamma-butyrolactone, tetrahydrofuran, dimethyl formamide and dimethylsulfoxide. These solvents may be used individually or in admixture witheach other, or with other solvents, such as ethylene carbonate,acetonitrile and methyl or butyl formate.

In addition to lithium, light metal anode may be formed of sodium,magnesium, calcium, beryllium, aluminum, and the like.

For purposes of explanation of the characteristics and function ofbarrier 18, the cell of FIG. 1 will be discussed as having anelectrolyte comprised of lithium perchlorate (LiClO4) in tetrahydrofuran(THF). The ionic constituency of this electrolyte is the lithium cationLi+ and the perchlorate anion ClOf. Also present in the electrolyte arepolysulfide anions Sx2, x=29, this being a soluble cathodic dischargeproduct, and sulfur anions S=.

In operation of the cell, it is of course essential that ionicconductivity be maintained in the electrolyte by the migration ofcations to cathode 14 from anode 12. It is further essential that freeelectrolyte ow be reduced to a negligible rate. These considerationsdemand that barrier 18 have at least the characteristic ofmicroporosity, i.e. that the barrier provide a common physicalimpediment to ion transport.

Barriers having only this characteristic, such as are disclosed in thereferenced U.S. 3,413,154, do not particularly discriminate againstmigration of any ionic species in the electrolyte. Thus, the cathodicmass comprised of the sulfur electrode 14, and the dissolved polysuliideundergoes some measure of self-discharge with the polysulide anionsbecoming distributed throughout the electrolyte rather than being connedto the cathode side of the barrier. Attending such migration of thepolysulde anions is a further cell reaction in which the lithium anodebecomes passivated over an extended period of time.

In contrast to barriers providing only a characteristic microporosity,barrier 18 of the cell of FIG. 1 includes the further property ofpermselectivity, i.e. the barrier permits passage of some ionic speciesand not others.

It has been found that a barrier having suitable microporosity andpermselectivity for use in a light metal-sulfur organic electrolyte cellmay consist of commercially available ion exchange membranes heretoforein widespread use for demineralization, dealkalization and softening ofWater and like purposes in connection with laqueous mediums. Suchmembranes are synthetic organic resins comprising cross-linkedpolyelectrolytes having a large number of ion active groups attachedthereto. In general these membranes comprise a cross-linked polystyrenepolymer which is sulfonated or carboxylated to develop the cationicexchange membrane or which is aminated to develop the auionic exchangemembrane. Barrier 18 of FIG. 1 is illustrated as a cationic membranecomprised of rigid molecules R of the insoluble polymer each having afunctional sulfonated group S03 attached thereto, the membraneincorporating further a required lithium counter cation Li+. The lithiumcounter cation in contrast to the rigid molecule and attached functionalgroup, is bound loosely in the membrane and is capable of ready movementfrom the membrane into an associated solution. One characteristic formembrane selection is that the concentration of counter cations in themembrane be considerably greater than the concentration of cations inthe cell electrolyte. Thus, in the cell of FIG. l, barrier 18 preferablyhas a concentration of lithium ions of approximately ten times theconcentration of lithium ions in the lithium perchlorate tetrahydrofuranelectrolyte.

As a result of these relative concentrations and the loose containmentof the counter cations in the membrane, upon assembly of the cell, thelithium counter cations contained in barrier 18 tend to egress from themembrane into the electrolyte. As a result, the membrane acquires a netnegative charge and there is established the Donnan potential dilerencebetween the membrane and the electrolyte, a primary prerequisite forpermselectivity. The membrane is thereby rendered effective toelectrostatically repel migrating polysuldes which also bear a negativecharge.

Barrier 18 is a cation permeable electronegative membrane and has noelectrostatic effect on the lithium ions in the electrolyte or othercations in the cell. Thus, cell cations encounter no other diculty inmigration to cathode 14 other than that posed by the microporouscharacter of barrier 18. In this connection, it has been clearlyestablished by migration testing and utilization testing that thecationic membrane is etective to reduce if not totally inhibit migrationof the polysulde anions from cathode to anode and that the membrane isnot effective in inhibiting migration of cell cations to the extent thatcell performance is degraded during its extended operating life whencompared with the performance of the lter paper equipped cells duringtheir shorter operating life.

Migration testing In order to evaluate the eifect of the ion exchangemembrane cell barrier on polysulde migration an H-cell was constructed.One half of the cell was lled with a colorless 0.1 molar solution oflithium perchlorate in tetrahydrofuran. The other cell half was llerdwith a deep brown polysuliide solution constituted of excess lithiumsulde and sulfur in tetrahydrofuran. A cation exchange membraneavailable commercially as RAI P300 40/20, a product of RAI ResearchCorporation, was placed between the cell halves along Iwith an O-ringproviding a seal to prevent leakage. The two cell halves were thenclamped together. The distinct coloration of the constituents of each ofthe two cell halves permitted a ready observation of polysuldediffusion. During an observation period of six weeks, no color change ofcell halves rwas noted, supporting a conclusion that there was nodiusion of the polysulde anions through the cation exchange membrane.

Utilization testing In a second experimental series, the elect of an ionexchange membrane barrier on the utilization of a sulfur cathode wasstudied. The extensive improvement in utilization is evident in thegraphic showing of results in FIG. 2. In this series of tests, threecells were constructed, each including a silver reference electrode, alithium anode and a cathode prepared by illing a stainless steel screen(500 mesh) bag with a 1:1 mixture of sulfur and graphite pressed at 2000pounds per square centimeter. The silver reference electrode wasincluded solely for voltage measurement purposes and is of course notemployed in practical cells. In each cell the electrolyte consisted of aone molar solution of lithium perchlorate in tetrahydrofuran. In onecell (A) the barrier was a tilter paper separator. Current density wasone ma./cm.2. It will be seen in FIG. 2 that the cell provided an opencircuit voltage of about 3.0 volts and an initial operating voltage of2.4 volts under load. Cell utilization eiiiciency of 20% may be seen inFIG. 2.

In a second cell (B) the barrier was constituted of one layer of the RAIP300 40/20 ion exchange membrane. Cell current density was one ma./cm.2.As indicated in FIG. 2, the cell provided an open circuit voltage ofabout 3.0 volts and an initial operating voltage of 2.1 volts underload. As shown in FIG. 2, cell utilization eiliciency was 40%.

In a third cell (C) the barrier member was constituted of two layers ofthe RAI P300 40/ 20 ion exchange membrane. Current density was 0.5ma./cm.2. The cell again provided an open circuit voltage of about 3.0volts and ar. initial operating voltage of 1.7 volts under load. Cellutilization eiciency of almost 60% may be seen in FIG. 2.

The results of this testing of cells (B) and (C) indicate thatimprovements in utilization of twofold and threefold may be achieved bythe use of cationic exchange membranes in lithium-sulfur organicelectrolyte cells. It appears that the only cell characteristic which isdegraded in this improvement is cell output voltage under load whichdecreases by reason of increased internal cell resistance. In cell (B)this decrease in cell output voltage amounts to approximately Thisutilization testing establishes further, particularly when taken inconjunction with the migraiton testing above, that the presence of theion exchange membrane in the cell, while eective to inhibit migration ofthe polysulde anions, does not inhibit migration of the lithium cationnecessary tfor cell operation.

It will be evident that the nature of the ion exchange membrane, i.e.cationic or anionic, employed in the lithium-sulfur cell is dependentupon the nature of the ionic species whose migration is desired to beinhibited. While the discussion has centered upon the soluble polysuldesand their retention in the vicinity of the cell cathode, it is withinthe contemplation of the inventon to employ an anionic cxchnage membraneas a cell separator where the constituency of the cell gives rise to thepresence in the cell of a cation whose migration is detrimental to cellshelf life and utilization. Similarly, it is within the contemplation ofthe invention to employ as the barrier member of the lithium-sulfurorganic electrolyte cell a bipolar or amphoteric ion exchange membrane,i.e. a membrane having anionic and cationic components.

The above-discussed requisite that the ion exchange membrane contain arelatively high concentration of counter anions compared to theconcentration of anions in the electrolyte, is generally indicated bythe exchange capacity of the membrane which is defined as the number ofequivalents of xed ionic groups in the resin per unit weight (dry) ofthe resin. For commercially available` ion exchange membranes, theexchange capacity is normally determined in aqueous medium `for whichapplication of the membrane is designed. Thus, in the present use ofsuch membranes, in a non-aqueous medium, it should be noted thatmembrane exchange capacity is generally reduced from that specified bythe manufacturer. Reduction of exchange capacity in the order of 40% maybe expected in the organic electrolytes.

A further consideration in the selection of a barrier member -for alithium-sulfur organic electrolyte from the commercially available ionexchange membranes is the compatibility of the membrane with theelectrolyte solvent. In this regard membrane suitability may bedetermined by placing the selected membrane in the organic solvent andobserving the elect of the solvent thereon. Solubility of the membraneresin or its backing member in the solvent renders the membraneunsuitable for use.

One further characteristic of importance to the selection of membranesfor use in the present invention is membrane conductivity. In thisconnection, it should be noted that the low conductivity (highresistivity) membranes produce undesirably high cell IR drops. Thus,higher conductivity membranes are preferably employed. It should benoted also that membrane conductivity may be improved by increasing itsexchange capacity. Thus, increasing the lithium ion concentration in amembrane will render useful an otherwise unsuitable membrane.

While this invention has been described in connection with the aboveparticular cell structure, this is intended in a descriptive and not alimiting sense. Such changes and modifications as will be evident tothose having ordinary skill in the art to which the invention appliesare within the contemplation of the invention.

What is claimed is:

1. A high energy density electric cell comprising a light metal anode, acathode the active material of which is sulfur, an organic electrolytecomprising an organic solvent containing an inorganic salt dissolvedtherein and microporous permselective barrier means separating saidcathode and said electrolyte, said barrier means comprising an ionexchange membrane comprising a cross-linked polystyrene polymer.

2. The electric cell claimed in claim 1 wherein said ion exchangemembrane is cationic.

3. The electric cell claimed in claim 1 wherein said cathode includes aparticulate material of higher electrical conductivity than sulfur.

4. The cell claimed in claim 1 wherein said electrolyte comprises anorganic solvent selected from the group consisting of propylenecarbonate, gamma-butyrolactone, tetrahydrofuran, dimethyl formamide anddimethyl sulfoxide, said solvent containing a salt.

5f The cell claimed in claim 4 wherein said salt includes a cationselected from the group including ammonium and light metals.

6. The cell claimed in claim 5 wherein said salt includes an anionselected from the group consisting of tetrafluoroborate,tetrachloroaluminate, perchlorate and chloride.

7. The cell claimed in claim 1 wherein said anode is lithium.

8. The cell claimed in claim 7 wherein said electrolyte comprisestetrahydrofuran containing lithium perchlorate.

9. The cell claimed in claim 7 wherein said anode is magnesium.

10. The cell claimed in claim 7 wherein said anode is aluminum.

References Cited UNITED STATES PATENTS 3,531,328 9/1970 Bro etal136--100 R 3,532,543 10/1970 Nole et al 136--6 LN 3,639,174 2/ 1972Kegelman 136-6 LN 3,185,590 5/1965 Mayer et a1. 136-6 LN 2,861,11611/1958 Grubb, Jr. 136--153 2,913,511 11/1959 Grubb, Ir 136-120 FC3,393,092 7/1968 Shaw et al 136-6 LN 3,376,168 4/1968 Horowitz 136-1463,393,093 7/1968 Shaw et al 136-6 LN 3,415,687 12/1968 Methlie 136-100 R3,413,154 11/1968 Rao 136-100 R ALLEN B. CURTIS, Primary Examiner C. F.LE FEVOUR, Assistant Examiner U.S. Cl. X.R. 136--100

